Method of producing discharge display device

ABSTRACT

The present invention discloses a method of producing a discharge display device which enables a LaB 6  cathode to be formed by a thick-film printing method. The method of the present invention comprises the steps of applying a paste prepared by mixing LaB 6  powder with alkali glass powder in a proportion of 20-40 wt. % with respect to the LaB 6  powder to a base electrode, burning the paste, and activating the paste by gas discharge with large current after an exhaustion step to form a LaB 6  cathode on the base electrode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of producing a discharge displaydevice and more particularly to a method of forming a LaB₆ cathode forthe discharge display device.

2. Description of the Prior Art

Recently, development of discharge display devices, especially directcurrent type XY matrix discharge display panels termed plasma displaypanel or PDP has been promoted. In such a discharge display panel,Nickel (Ni) is usually used as an anode and a cathode. However, Ni hasinsufficient resistance against discharge sputtering, and therefore a Nicathode deteriorates in several seconds of operation. To cope with this,mercury (Hg) has been sealed in the discharge display panel anddeposited on a surface of the electrode to suppress sputtering.

On the other hand, a direct current type discharge display paneldeveloped by the present inventors employs a unique driving system, thatis, a trigger discharging system, and when it is applied to an XY matrixpanel with a large capacity, it is necessary to provide dischargecharacteristics, (i.e., the characteristics of trigger discharge andmain discharge) of each display cell uniform to a certain degree.However, in a discharge display panel having mercury (Hg) sealedtherein, a non-uniform distribution of the mercury commonly occurs dueto change on tanding, and it is difficult to retain uniform dischargecharacteristics for a long time. For this reason, it is important toprovide a discharge display panel in which no mercury is sealed.Further, for example, where a discharge display panel is to be used in aclosed room such as a cockpit, mercury should not be used inconsideration of danger.

Further, in the XY matrix type discharge display panel, it is generallyimportant to attain reduction in power consumption, long life, highdischarge efficiency and reduced driving voltage, etc. Meanwhile,lathanum boride (LaB₆) has been noticed as a cathode material. LaB₆ islow in its discharge holding voltage, and is stable in physical andchemical properties, thus meeting the above-mentioned requirements.

However, a LaB₆ cathode has not yet reached practical use for the reasonthat production employing a thin-film evaporation method or a plasmaspraying method is complicated and results in increase in cost.Particularly, it is difficult to form a relatively uniform electrodewith a large capacity and a large screen. Another reason is that theelectrode cannot be formed in connection with the other panel structureby a thick-film printing method with a low cost.

In a case where the LaB₆ cathode is intended to be formed by thethick-film printing method, it is generally burnt in an atmosphere ofnitrogen of N₂ at 800° C.-900° C. after printing and application.However, as a substrate of the discharge display panel is glass,temperature is permitted to be raised up to about 600° C., and as astructure such as the other electrodes and a barrier is oxide, a burningstep is usually carried out in the air. For these reasons, it isdifficult to form the LaB₆ cathode. In addition, LaB₆ has a high meltingpoint of about 2300° C., and therefore it cannot be sintered at atemperature of about 600° C., with a result that resistance afterformation of the cathode is disadvantageously increased to 10⁹, andmore. In the case that the thick-film printing method is adopted, abinder substance such as frit glass is generally mixed with LaB₆ powderso as to obtain a bonding strength between particles of the LaB₆ powder.However, it is considered not possible to use a mixture of such glassbinder with LaB₆ powder, due to the resulting high resistance afterformation of the LaB₆ cathode.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide amethod of producing a discharge display device which enables a LaB₆cathode to be formed by a thick-film printing method.

In accordance with the present invention, a method of producing adischarge display device comprises the steps of applying a pasteprepared by mixing LaB₆ powder with alkali glass powder in a proportionof 20-40 wt. % with respect to the LaB₆ powder, to a base electrode,burning the paste, and then activating the paste by gas discharge withlarge current after an exhaustion step, to form a LaB₆ cathode on thebase electrode.

According to the method of the present invention, it is possible toeasily form a LaB₆ cathode by the thick-film printing method, and obtaina discharge display device having improved characteristics such as lowdriving voltage, long life and high discharge efficiency.

In other words, it is possible to easily form the LaB₆ cathode by aso-called thick-film printing method by the steps of applying andprinting the LaB₆ paste, and subsequently effecting activation treatmentby gas discharge with large current.

Further, since the glass binder is contained in the LaB₆ paste, a LaB₆cathode having a large adhesive strength may be obtained. Additionally,since an alkali glass powder having ionic conducting property is used asthe glass binder, and the alkali glass powder is mixed in a proportionof 20-40 wt. % with respect to LaB₆ powder, the activation treatment maybe satisfactorily effected.

In accordance with the invention, it is possible to produce a dischargedisplay device with a large capacity and a large area. Further,formation of the LaB₆ cathode is simplified as compared with anevaporation method, etc., thus reducing cost.

In this connection, the possibility of formation of the LaB₆ cathodeimparts the following advantages. That is, driving voltage in thedischarge display device may be lowered, and accordingly circuit costmay be reduced by using IC. Power consumption may be reduced. Owing tothe fact that LaB₆ is superior in anti-sputtering performance, and isstable in physical and chemical properties, and sputter voltage isdecreased due to the low driving voltage, life of the discharge displaydevice is extended. High luminance may be achieved by improvement indischarge efficiency and reduction in power consumption. Further,application of this type of discharge display device is expanded owingto elimination of mercury.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial perspective view of an exemplary discharge displaydevice employable in accordance with the present invention;

FIG. 2A to 2C are exemplary illustrations, in cross-section, offormation of LaB₆ cathode according to the present invention; and

FIG. 3 is a graph showing change in a holding voltage during activationtreatment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

First, an exemplary discharge display device employable in accordancewith the present invention will now be described with reference to FIG.1, in which the discharge display device is applied to a direct currenttype discharge display panel of a trigger discharge system. There, adischarge panel 1 comprises a front glass substrate 2, a rear glasssubstrate 3, anodes 4 and cathodes 5 of XY matrix shape. The anodes 4are partitioned from each other by insulating barriers 6. On the rearglass substrate 3, trigger electrodes 8, formed of aluminum (Al) forexample, are arranged in parallel relation with the cathodes 5 throughan insulated dielectric layer 7 under the cathodes 5.

The display panel 1 is manufactured in the following manner. First, theanodes 4 and the insulating barriers 6 are formed on the front glasssubstrate 2 by a thick-film printing method. Similarly, the triggerelectrodes 8, the insulated dielectric layer 7 and the cathodes 5 aresequentially formed on the rear glass substrate 3 by the thick-filmprinting method. Each of these constitutional parts is burnt afterprinting. Then, both the glass substrates 2 and 3 are oppositelyarranged with the anodes 4 and the cathodes 5 cross at a right angle,and are frit-sealed about the periphery. Thereafter, heating exhaustion,gas sealing (e.g., Ne-Ar ga) and final sealing are carried out tocomplete the display panel 1.

In such a discharge display panel 1 as obtained above, a driving voltageis selectively applied to the anodes 4 and the cathodes 5 to generatedischarge luminescence at cross-points between the selected anodes 4 andcathodes 5, thereby effecting display in a linearly sequential manner.Especially, in this display panel 1, a trigger voltage is applied to thetrigger electrodes 8 prior to effecting of discharge between the anodes4 and the cathodes 5 to induce a wall voltage on a portion of theinsulated dielectric layer 7 corresponding to the trigger electrodes 8and effect momentary discharge between the insulated dielectric layer 7and the selected cathodes 5. As a result, a gas space along the cathodes5 is ionized, so that subsequent discharge between the selected anodes 4and cathodes 5 may be easily effected.

The present invention is directed to a method of forming the cathodes 5in the discharge display panel by the thick-film printing method. Apreferred embodiment of the present invention will be described below.

In the present invention, LaB₆ paste consisting of LaB₆ powder,inorganic binder and suitable vehicle (solvent) is preliminarilyprepared. The LaB₆ powder as a raw material is selected in such a mannerthat an average particle size thereof is to be not more than several μm,preferably 1-3 μm, and powder having the average particle size of notless than 5 μm is to be contained in a proportion of not more than 5%with respect to the total amount of LaB₆ powder. As the LaB₆ powder issufficiently unbound from its sintered state in general, it is furtherfinely pulverized with a ball mill. As the inorganic binder, an alkaliglass is used, because a certain degree of ionic conduction is requiredin a subsequent activation step. A fine powder of the alkali glass isadded in the amount of 0.2-0.4 parts by weight with respect to 1 part byweight of the LaB₆ powder. If the amount of the alkali glass fine powderis too small, activation is rendered non-uniform, while if it is toomuch, the activation is difficult to effect.

As shown in FIG. 2A, first a conductive paste such as Nickel (Ni) pasteis applied and printed along a cathode pattern to be formed on theinsulated dielectric layer 7 formed on the rear glass substrate 3, andis burnt to form Ni base electrodes 10. The Ni base electrodes 10 serveas a lead wire for supplying current to a LaB₆ cathode which will besubsequently formed.

Then, as shown in FIG. 2B, the LaB₆ paste as mentioned above is printedon the Ni base electrodes 10, and is then burnt in a dry air at500°-600° C. for 30 min. to for a LaB₆ layer 11. The resistance afterbeing burnt is rendered high, namely, not less than 10⁹ Ω.

Then, the front glass substrate 2 on which the nodes 4, formed of Ni forexample, and the barriers 6 are formed as mentioned above and the rearglass substrate 3 are frit-sealed around the edges, and heatingexhaustion, sealing of desired gas and final sealing are carried out.Thereafter, a predetermined voltage is applied between the anodes 4 andthe Ni base electrodes 10 to effect activation treatment by gasdischarge with a large current (cathode forming). With this activationtreatment, no glass becomes present on the LaB₆ layer 11 (socalleddischarge surface), and LaB₆ itself is exposed to the discharge surface.Furthermore, sintering of LaB₆ powders occurs owing to a local thermaleffect to make the surface of the LaB₆ layer in a fused and boundcondition. As a result, electrical continuity is provided to reduce theresistance in the LaB₆ layer. Thusly, as shown in FIG. 2C, a LaB₆cathode 12 is formed on the Ni base electrode 10.

A current density during activation is about 2-5 A/cm² . FIG. 3 showschange in a holding voltage during activation, provided that theactivation treatment is carried out at a current density of 3 A/cm² with0.5 sec ON-0.5 sec OFF set, As will be apparent from FIG. 3, at aninitial stage, a firing potential is high (200 V and over), anddispersion is large. However, as time is elapsed, the firing potentialis lowered and is stabilized in 2-3 hours. Further, dispersion becomessmall after about one hour has elapsed.

The holding voltage in a normally driving region after activation isabout 110 V. Comparatively, in case of Ni cathode; the holding voltageis about 150 V.

According to the method of the present invention, the LaB₆ paste isapplied and printed to the base electrode, and is burnt, thereaftercarrying out activation by gas discharge with large current after anexhaustion step, thereby permitting the LaB₆ cathode to be formed by aso-called thick-film printing method. Since the LaB₆ paste contains aglass binder, both the bonding strength between each of the LaB₆cathodes and the base electrode are large, and the LaB₆ cathodes are noteasily separated even if they are slightly rubbed during the fritsealing step. Furthermore, since the alkali glass having ionicconducting property is used as the glass binder, the subsequentactivation treatment may be securely effected. Additionally, since theLaB₆ paste layer is burned in the air at about 500°-600° C., the rearglass substrate is not damaged, and the other structures of oxide arenot badly influenced.

Although the preferred embodiment as mentioned above is applied to thedirect current type discharge display panel of trigger discharge system,it will be appreciated that the present invention is applicable toformation of the LaB₆ cathode for the other discharge display panels.

I claim as my invention:
 1. A method of producing a discharge displaydevice comprising the steps of applying a paste prepared by mixing LaB₆powder with alkali glass powder in a proportion of 20-40 wt. % of glasspowder with respect to the LaB₆ powder, to a base electrode, burning thepaste, and activating the paste by gas discharge with large currentfollowing an exhaustion step to form a LaB₆ cathode on said baseelectrode.
 2. The method of claim 1 wherein the paste is formed in dryair at a temperature of about 500°-600° C. for a period of about 30minutes.
 3. The method set forth in claim 1 wherein said large currentis in the range of 2-5 amps per square centimeters.